An easy/reversable solution would be: If you don't need the wheels, you could put some larger risers in those holes.

More difficult: assuming both legs are hollow, you could cut them near the top. Then you could try to find a small rod that fits into them that you can secure with some bolts drilled through. But be careful because it can affect the stability of the table.

The first solution is exactly what I'm hoping for! Do you know what I need to look for to get that done? I don't know what such a connection is called.

First, you will need to remove the wheels and see if they are bolts.

If they are bolts, measure their width to get their size and replace them with something like this. https://www.meubelpootjes.be/plastic-ronde-meubelpoot-6-cm-m8-k000381

If they are not bolts, see if you can replace them by removing the black inserts in the bottom beams.

I made two rectangles out of scrap 2 x4s and attached some wheels underneath. Cross piece to connect the two boxes at the proper width for the table legs. The drafting table sits atop the risers with two pieces of small 1/4 inch scrap on each rectangle to keep the table from shifting around. Had bought a children's drafting table when I was in art school because it was that much cheaper and I was an older student with a mortgage to pay. Works great, still use it despite having acquired a standard size drafting table. The wheels ended up being a great decision.

I thought the wheels had a click system, but they are indeed bolts. Dankjewel! Dit is de perfecte oplossing!

I'm going with u/dark-archon's idea for now, but yours will be the perfect option if I ever need the wheels on there. Thanks!

Graag gedaan. Goed de maat van de bouten nakijken, wss is het een een M8 of M10.

Het is inderdaad M8. Ik ben niet zo bekend met de maten en het leek even op 7mm, dus bedankt voor de extra info!